1. Field of the Invention
The objective of the present invention is an underwater objects passive tracking process and device.
The technical scope of the invention is the field of acoustic telemetry.
One of the principal applications of the invention is the possibility to know the trajectory followed by a marine mobile in real time, starting from the individual noises radiated by the latter, e.g. during self-propulsed equipment test phases in order to know the performances, particularly for torpedoes.
2. Related Art
Indeed, two families of trajectory tracking processes and devices are known:
the first family is the only one used today to establish fine trajectography, like in test centers or on crowded offshore sites; it groups active telemetry equipments comprising emitters located on the mobile, which produce acoustic waves in a given frequency or a frequency spectrum, according to given time intervals, thus allowing to easily identify them or to collect them by means of receivers uniformly and individually located, in a fixed way on the site. The received signals being processed, it is possible to deduce the position of the mobile at any time, and step by step, its trajectory. PA1 The other family of this invention is essentially used today for the passive detection of the other mobiles which might threaten a site or sub-marine missile; any suspect surrounding noise must be then identified, or we must try to identify the source and to determine if the direction of displacement of the latter does not converge towards the concerned site or missile; this alarm function has only rough actions, even if improvements have been brought, applications for a patent having been filed for some of them; such as N.degree. FR 2.504.275 filed on Apr. 15, 1981 and N.degree. FR. 2.614.427 filed on Apr. 24, 1987 by THOMSON CSF company concerning "Passive sound telemetry processes and systems using three sensors receivers in a line"; the three received signals are processed by taking the central receiver as a reference to compute the delays of the two others and to correlate the whole. PA1 at defined and known time intervals, the S1 and S2 signals coming from the two said sensors are collected; then they constitute a pair whose distance is known, the latter being selected lower than the distance separating them from the other hydrophones, and whose accurate direction of alignment is also known; PA1 one of said signals S1 is chosen as a reference; it is received at a time t1 from one of said hydrophones and a S2 signal supposed to be representative of the same sound wave is collected, this sound wave being emitted at the same original moment t0 by said object, and received from the other hydrophone at a moment t2 estimated depending on an estimated delay linked to the presumed direction "d" of the object at a given moment of measurement compared to the alignment of the pair of hydrophones; PA1 this second signal S2 is corrected according to the presumed speed of the object at instant t0, to take into account the Doppler's effect, and the here-above presumed data are corrected until the maximum value of a function of intercorrelation between signals S1 and S2 is obtained; PA1 then the operation of measurement and computing is started again after the next time interval with a defined and known duration, considering the correlated and corrected data of the previous time interval, which are modified by the increment estimated values, extrapolated according to the travelled trajectory of the object and computed by the previous time intervals.
The main disadvantage of these systems is the necessity of installing an emitter on the mobile, space requirement being a problem and then its performances are limited, with in addition the risks of failure or too low emission levels, perturbations of homing heads which may be possibly installed on the mobiles or on board the torpedoes, a complication of implementation procedures on sea, etc., without forgetting the problem of these emitters stock management.
Therefore, the processes and devices of that second family do not allow to know with accuracy the location of the localized missile to be positioned at any time in order to know the trajectory accurate characteristics, as it eliminate many factors, particularly the unsteadiness and anisotropy of the noises radiated by the latter; indeed, the combination of the noises produced by the shipped motor(s), the fluid flow around its shell, the cavitation of the propeller(s), the internal vibrations, etc., cannot be steady for long, nor homogeneous in all directions. This means that an observer motionless compared to the missile sees two different spectra at two different moments, and the same occurs for two observers placed in different points at the same moment.
Moreover, in active mode as well as in passive mode, in the two families of trajectory tracking processes and devices recalled here-above, the fact that the missile is in motion is always neglected; in that case, the signal received by an observer, originated from the mobile missile itself, or coming from a shipped emitter, is then modified by the Doppler's effect, to obtain correlated signals, they should be processed by being assigned a suitable Doppler's coefficient; but, by definition, the latter is unknown at the time of the initial detection.
Taking into account the inaccuracy of all the actions such as they are recalled here-above, and the heaviness and slowness of the correlation computing which then would not allow a tracking in real time with the known processes and devices, today this Doppler's effect is neglected, and thus it is impossible to know the searched trajectory with accuracy.
The stated problem is to define passive trajectography processes and devices for underwater polygons or sites, allowing to track a mobile travelling towards and among them, with a good accuracy, as regards geometric coordinates and the speed and acceleration, and taking into account the Doppler's effect provoked by its speed as well as the factors of anisotropy and unsteadiness of the noise radiated by the mobile itself.